Inorganic materials, such as glass-ceramics, which are transparent or translucent in the visible wavelength range and show high stability with regard to temperature fluctuations and which are utilized, for example, as hot plates, are known and commercially available. These glass-ceramics are colored by means of coloring oxides, such as MnO.sub.2, Fe.sub.2 O.sub.3, NiO, CoO, Cr.sub.2 O.sub.3, V.sub.2 O.sub.5, and CuO. The effect of these coloring oxides on coloration, i.e., the absorption in the visible wavelength region, is discussed in the prior art.
Thus, DE-AS 1,596,858 describes the effect of the individual oxides CoO, Cr.sub.2 O.sub.3, NiO, and Fe.sub.2 O.sub.3, as well as CoO in combination with MnO.sub.2 and CuO, on transmission in the visible wavelength region.
U.S. Pat. No. 3,788,865 examines the effect of combinations of two oxides selected from CoO, NiO, Fe.sub.2 O.sub.3, Cr.sub.2 O.sub.3, MnO.sub.2, and CuO on the transmission in the wavelength range from 400-700 nm. Moreover, the simultaneous effect of the three oxides NiO, CoO and Fe.sub.2 O.sub.3, as well as that of V.sub.2 O.sub.5, on the transmission is described. Although no measured results are provided, it is pointed out that the V.sub.2 O.sub.5 -containing glass-ceramic shows good transparency in the IR range.
German Patent 2,429,563 discloses the combined effect of the four oxides CoO, NiO, Fe.sub.2 O.sub.3, and MnO.sub.2 on the transmission. In the wavelength region from 700-800 nm, a transmission of above 70% is observed, dependent on the hue of transmitted light, while the IR transmission drops, for certain wavelengths, to below 10%.
U.S. Pat. No. 4,211,820 describes a brown glass-ceramic wherein the dye effect is obtained by TiO.sub.2 and V.sub.2 O.sub.5. Besides these oxides, only Fe.sub.2 O.sub.3 is contained therein in minor amounts as the coloring oxide. The characterization of the transmission is inadequate, and one can only speculate that the brown coloring is obtained by measuring the transmission on 5 mm thick specimens at .lambda.=800 nm. There is no data regarding transmission in the IR range.
Finally, EP 0 220 333 B1 discloses a transparent glass-ceramic containing high quartz mixed crystals which appears black in incident light and violet or brown to dark red in transmitted light. It is especially suited for the production of hot plates, wherein the change, caused by temperature stresses, in the linear thermal expansion coefficient and transmission is small. The transmission in the IR range can be variably adjusted between 800 nm and 2.6 .mu.m, and the glass-ceramic starting material has the following composition (in weight percent):
wherein the coloring is made possible by combining oxides selected from the group consisting of V.sub.2 O.sub.5, NiO, CoO, MnO.sub.2, Fe.sub.2 O.sub.3, and Cr.sub.2 O.sub.3. The glass-ceramic is converted from this glass-ceramic starting material by a heat treatment of from 680.degree.-920.degree. C.
As can be seen from the aforementioned patents, the effect of coloring oxides on transmission in the visible range of the h-quartz mixed crystal-containing glass-ceramics has been examined in detail. According to the art, it is possible, for example, to manufacture in a controlled fashion heatable plates having a thickness of about 4 mm, which appear opaque (black) in incident light and, in transmitted light, violet, brown, up to dark red. Due to these properties, heating elements used with a cooking surface or in similar applications are clearly visible during operation, while they are not visible through the cooking surface in the unused condition.
In contrast to the transmission in the visible wavelength region, the effect of the coloring oxides on transmission in the IR region of wavelengths higher than 2.6 .mu.m has not been studied in detail.
U.S. Pat. No. 4,057,434 describes an opaque glass-ceramic having a thermal expansion coefficient (20.degree.-700.degree. C.) of less than 15.times.10.sup.-7 /K, with excellent chemical stability, and an infrared transmission at a wavelength of 3.5 .mu.m through a polished plate having a thickness of 4.25 mm of more than 40%. The glass-ceramic has .beta.-spodumene as the single crystalline phase consisting of (in weight percent on oxide basis) 2.5-4.5%. Li.sub.2 O; 0.75-3.5%, ZnO; 17.5-21%, Al.sub.2 O.sub.3 ; 65-71%, SiO.sub.2 ; and 3.5-6%, TiO.sub.2, and being essentially devoid of alkaline earth oxides and alkali oxides, except for Li.sub.2 O and ZrO.sub.2.
U.S. Pat. No. 4,575,493 relates to an infrared-permeable glass having a thermal expansion coefficient of less than 4.24.times.10.sup.-6 /.degree. C., measured at 25.degree.-300.degree. C., consisting of (in mol%) ZnO, 15-30; Al.sub.2 O.sub.3, 2-10; Ta.sub.2 O.sub.5, 2-15; and GeO.sub.2, 40-75.
Of the commercially available glass-ceramics, e.g., by Corning, Corning Code 9632, it is known that glass-ceramics colored with V.sub.2 O.sub.5 have a very high transmission in the IR range of 1-2.6 .mu.m, namely, about 80% for specimens with a thickness of 4 mm Also, CERAN.RTM. "HIGH TRANS".TM., glass-ceramic cooking surfaces sold by Schott colored with V.sub.2 O.sub.5, exhibit a very high IR transmission of about 80% for 3 mm thick specimens (up to .lambda.=2.6 .mu.m).
In the wavelength range of 2.7-3.3 .mu.m, however, IR transmission drops to very low values in all glass-ceramics presently on the market, for example, to below 5% at a wavelength of 2.8 .mu.m for a glass-ceramic plate with a thickness of 3 mm.
For example, when hot plates are used as a cooking surface, IR transmission is one of the determining variables for good efficiency of the cooking system, i.e., short warm-up times and low energy consumption. The most common heating elements with open heating coils radiate in the range from 2.7-3.3 .mu.m with 80-95% of their maximum emission. However, precisely this range is absorbed by the materials presently on the market. Therefore, the radiated energy is not directly available at the bottom of the pan but rather only by way of thermal conduction or secondary radiation of the heated cooking surface.
It is shown in German Patent 2,437,026, as well as "Schott Information 2/84", that it is very difficult to optimize a cooking system, especially due to varying configurations of cooking vessel surfaces, but that such optimization is possible, in substantial part, by changing the IR transmission of the cooking surface.
Also, the development of heating units for glass-ceramic is ongoing. It is definitely possible, for example, to envision future heating systems exhibiting a different radiation characteristic from present the heating units and/or constructed of several heating zones with differing radiation temperatures and/or characteristics.
These heating systems are to be taken into account, as well, in the development of an optimized glass-ceramic.